The invention relates to a recording or reproducing unit with means for compensating the unbalance of plate-shaped information media, a so-called autobalancer being provided neither in the disc turntable nor in the drive motor.
Devices denoted as so-called autobalancers for the purpose of automatically compensating an unbalance, which proceeds from the information medium, for example from the CD, the disc turntable and, as the case may be, from the drive motor, are generally known. Such autobalancers became necessary with the continuously rising rate of rotation in CD ROM drives, in order to be able at higher speed to read from and/or write to the information media originally provided for lower speeds. The reason for the increase in the rate of rotation is to be seen, in particular, in the reduction in the access time and in the increase in the data rate provided. With the rise in the rate of rotation, which is already a high multiple of the simple scanning speed, it has become necessary, for the purpose of reliably guiding the scanning beam on the information track, to counteract the centrifugal forces and/or vibrations produced by eccentricity and irregular mass distribution. An autobalancer provided in the disc turntable for the information medium comprises a circularly running groove in which there are provided for the purpose of unbalance compensation a plurality of balls which rotate with the disc turntable and are mutually aligned in the circularly running groove in a fashion counteracting the unbalance in the event of the occurrence of an eccentricity.
Since, as a rule, the disc turntable forms a structural unit with the drive motor, it is, for example, not possible to combine a cost-effective drive motor of another supplier with an autobalancer. Known autobalancers are arranged on the scanning side of the information medium in the disc turntable or in the drive motor. Consequently, the outside diameter possible for an autobalancer is limited by the inside scanning diameter of the information medium. In order to compensate a prescribed unbalance, an overall height is therefore required which is determined by the diameter of the balls to be used in the autobalancer. Moreover, the response of the autobalancer is to be improved in order already to ensure the effectiveness of the information medium when it has a slight unbalance, and to reduce vibrations of the information medium which are required for the functioning of the autobalancer.
It is the object of the invention basically to reduce the overall height required for the autobalancer, to improve its response and to create an autobalancer which is very largely independent of the drive motor and disc turntable for the information medium.
This object is achieved with means specified in independent claims, and advantageous refinements and developments are specified in dependent claims.
One aspect of the invention is, despite the limited possibilities for configuring known autobalancers, to find a solution corresponding to the requirements mentioned in the object.
The invention is based on the principle of configuring the autobalancer to be independent of the scanning and drive sides of the information medium by virtue of the fact that it is arranged not on the scanning side of the information medium, but in the hold-down which retains the information medium on the disc turntable.
Since a groove in which the balls of the autobalancer are aligned relative to one another is to be arranged as centrally as possible relative to the driving axis, it was firstly to be assumed that it would not be possible to achieve comparable results with an autobalancer provided in the hold-down. Surprisingly, it has emerged that the advantages which result from the arrangement of the autobalancer in the hold-down outweigh the disadvantages of an enlarged tolerance chain between autobalancer and drive axis. The principle of an autobalancer consists in that an unbalance with a phase shift of 180 degrees between the vibration emanating from the unbalance of a system and the vibration of the balancer is compensated. The compliant support of the vibrating configuration is therefore of great importance. Arranging the autobalancer in the hold-down increases the distance from the clamping point of the vibrating system, with the result that the information medium experiences a lesser deflection in conjunction with a comparable deflection of the autobalancer. On the other hand, this also means that an amplitude of the vibrations which is required for the mode of operation of the autobalancer is reached comparatively earlier with reference to the autobalancer and with lesser vibration of the information medium. This means that arranging the autobalancer in the hold-down improves the response of the autobalancer, and that automatic compensation is already achieved with a lesser unbalance of the information medium.
Further advantages of the arrangement of an autobalancer in the hold-down consist in that its diameter is no longer limited by the inside scanning diameter of the information medium. For the purpose of reducing the height required for the autobalancer, an autobalancer is proposed which has in the groove of the autobalancer discs which fulfil a function comparable to the known balls. The weight of the compensating means can be reduced owing to the larger diameter of the circularly running groove, in which the means for compensating the unbalance are arranged. Given a larger diameter of the circle formed by the groove, it is therefore advantageously possible to use smaller balls or discs to compensate an unbalance, and to reach a nominal speed more quickly with a lesser mass. The use of a disc as means for compensating the unbalance in an autobalancer is advantageous, since both a lesser overall height and a lesser degree of running noise are achieved. By comparison with discs, balls cause greater running noise, since they run around several times and strike one another in the groove, as a rule, during acceleration and braking. Because of the greater friction, discs come to a standstill comparatively earlier. The disadvantage that discs rotate less because of the higher friction, and therefore initially appear less suitable as compensating means than balls is eliminated by using discs which have a projection with a lesser diameter. The bearing and/or friction surface is thereby reduced. According to one design, a disc provided as means for compensating unbalance is configured in such a way that it consists of two materials, the material in the inner region, which forms the projection, preferably being a plastic having good surface slip, and the material in the outer region being a heavy material, such as steel, lead or brass, for example. In order to ensure an adequate bearing surface, a ratio of inside to outside diameter in the range from 30 to 50% is provided. The lateral projection in the inner region of the disc is selected to be in the range from half a tenth to five tenths of a millimetre, and approximately two millimetres are provided as the thickness of the disc. However, in principle the disc can also be designed in one piece and symmetrically.
The autobalancer provided in the hold-down is very largely independent of the drive motor and disc turntable for the information medium, with the result that it is advantageously possible to combine different drive systems corresponding to the requirements with the automatic compensating device for rotating information media.